The present invention relates to particular polymer compositions based on fluoropolymers, making it possible to produce pipes or other articles capable of withstanding extremely severe conditions of use, such as those encountered in the offshore petroleum industry. It also relates to pipes and other articles manufactured from these compositions.
The materials used in the exploitation of petroleum deposits situated at sea are subjected to extreme conditions, in particular the pipes employed for conveying the hydrocarbons thus extracted. In fact, hydrocarbons are generally conveyed at high temperature (of the order of 135xc2x0 C.) and high pressure (for example 700 bars). Acute problems of mechanical strength and thermal and chemical resistance of the materials arise, therefore, when the plants are operating. To these are added other requirements before or after the use: thus, during their installation or their removal (unreeling-reeling), the pipes may be subjected to impacts, which they must withstand down to very low temperatures (for example xe2x88x9235xc2x0 C.), and to high distortions. A deformability of at least 7% is considered to be necessary to allow an (un)reeling which is not detrimental to the pipes. Finally, it is important that the properties of the pipes should remain virtually constant with the passage of time, in order to provide them with a long lifetime and possibly to enable them to be reused.
To try to meet all these short-term and long-term requirements, various types of pipes have already been proposed, generally comprising one or a number-of metal components guaranteeing the mechanical rigidity, for example a spiral-wound steel tape, as well as various layers based on polymeric compositions ensuring the leak-proofing and the thermal screening in particular. These polymeric compositions may be, for example, polyethylene-based, but this choice restricts the temperature of use of the pipes to less than 100xc2x0 C. They can also be based on fluoropolymers such as PVDF (polyvinylidene fluoride), which increases the maximum temperature of use and gives them an excellent chemical resistance. However, PVDF is very rigid and must therefore be plasticized, and this then presents the problem of the extraction of plasticizers by the hydrocarbons conveyed, which presents the risk of gradually resulting in a loss of the advantageous properties brought about by the plasticization, thus restricting the lifetime of the pipes and their reuse.
Finally, additional requirements appear when the manufacture of the pipes in question is envisaged. Thus, it is obviously desirable that the workability of the polymeric compositions should be as good as possible and that they should therefore be of moderate viscosity. However, in the case of pipes comprising a spiral-wound steel tape it is desirable that the production of a polymeric coating in contact with this spiral-wound tape should not be excessively perturbed by the presence of interstices between the turns. In fact, during the manufacture of the pipes projections of resin (xe2x80x9csaggingxe2x80x9d) may be produced in these interstices, constituting inhomogeneities and therefore potential failure initiators. It is preferable for this purpose that the composition employed should not have a viscosity that is too low.
Thus, in Patent Application EP 166,385 (Furukawa Electric Co. Ltd.) a solution which overcomes some of these problems is proposed. This document describes a multilayer pipe of particular structure capable in particular of being employed in the petroleum industry, comprising inter alia a spiral-wound steel tape and an adjacent layer consisting of a polymeric composition of specified properties, based on a PVDF copolymer. A copolymer obtained from VF2 (vinylidene fluoride) containing from 5 to 10% (molar) of HFP (hexafluoropropylene) is recommended because it improves the flexibility of the pipes thus formed. However, no attention is paid to the mechanical properties at low temperature. To prevent the interstices between the metal turns giving rise to the problems described above (sagging) it is furthermore recommended to provide an additional layer of plastic tape between the metal turns and the above-mentioned polymeric layer.
In Belgian Patent 832,851 (Dynamit Nobel A.G.), which also relates to a PVDF-(elastomeric) fluoro-copolymer (for example VF2-HFP) mixture, in order to give it high resilience and elongation at break without excessively affecting its heat resistance (the Vicat point), it is strongly recommended to crosslink the elastomer and optionally then to postcure the articles thus produced. These operations naturally require the use of special additives (crosslinking agents, vulcanization accelerators) and additional stages of manufacture. Furthermore, this document advises against exceeding a copolymer weight content of more than 30%.
The present invention is aimed at providing a polymeric composition with improved mechanical properties at low and elevated temperatures, which in the course of use retains a flexibility and a resilience which are sufficient in the service temperature range and which furthermore dispenses with the use of an additional layer of plastic tape intended to prevent the sagging and with resorting to crosslinking.
In the present case the invention relates to polymer compositions based on PVDF homopolymer and on fluorocopolymer(s), comprising, by weight:
(A) approximately from 60 to 80% of at least one PVDF homopolymer;
(B) approximately from 20 to 40% of at least one thermoplastic copolymer of vinylidene fluoride (VF2) and of at least one other fluoromonomer, present in this copolymer in weight proportions of approximately 5 to 20%;
(C) approximately from 5 to 20% [relative to the total weight of the polymers (A) and (B)] of at least one monomeric or polymeric plasticizer.
More precisely, the compositions of the invention comprise a mixture of VF2 polymers which contains, by weight:
(a) at least one PVDF homopolymer (A), in the proportions of at least approximately 60%, preferably of at least 65%, and not exceeding approximately 80%, preferably 75%; and
(b) at least one thermoplastic copolymer (B) of VF2 and of at least one other fluoromonomer (comonomer), this other monomer being present in this copolymer in weight proportions of at least approximately 5%, preferably of at least 8%, and not exceeding approximately 25%, preferably 20%, particularly preferably 16%. Fluorocomonomers which give very good results are HFP (hexafluoropropylene) and CTFE (chlorotrifluoroethylene). HFP gives excellent results. TrFE (trifluoroethylene) can also be employed. The weight content of this copolymer (B) in the compositions is at least approximately 20%, preferably at least 25%, and does not exceed approximately 40%, preferably 35%; as well as
(c) at least one plasticizer (C) in proportions of approximately 5 to 20%. All these proportions are expressed relative to the total weight of the polymers (A) and (B).
By virtue of the incorporation, according to the invention, of the copolymer (B) and of the plasticizer (C) in the homopolymer (A) a plasticizing effect is observed which is permanent at ambient and higher temperatures and, furthermore, an improvement in the mechanical properties at low temperatures. To this end, it is indispensable that the copolymer (B) should be thermoplastic (that is to say, at least in the present context, semicrystalline), and nonelastomeric. An xe2x80x9celastomericxe2x80x9d copolymer is intended to denote, as defined by the ASTM in the Special Technical Publication No. 184, a material that can be stretched, at ambient temperature, to twice its initial length and which, once released, quickly recovers its initial length to within 10%. Although it may appear surprising that the addition of a thermoplastic copolymer should lead to better plasticization than the addition of an elastomeric copolymer, the examples given below confirm that only the use of a thermoplastic copolymer (B) induces a significant plasticization effect, in contrast to the use of an elastomeric VF2 copolymer. It is thus observed that an elastomeric copolymer of this type is immiscible with PVDF, gives rise only to the improvement in the impact strength and gives a two-phase system (nodules of copolymer within a rigid homopolymer matrix). On the other hand, the compositions according to the invention can be considered as the result of a cocrystallization, and are single-phase. An unexpected and particularly important advantage of such a mixture of homopolymer (A) and of copolymer (B) is that it exhibits a synergistic effect when compared with a simple copolymer insofar as its melting temperature and its creep resistance at high temperature (reflected, for example, in its Vicat point) are concerned. It appears, in fact, that the melting temperature is markedly higher in the case of a composition according to the invention than in that of a copolymer alone, at equal overall comonomer content.
An additional condition which the thermoplastic copolymer (B) described above must preferably fulfil concerns its molecular weight or, more conveniently, its melt index.
The copolymer (B) of the composition according to the invention preferably has a melt index (MFI measured at 230xc2x0 C. under 10 kg) lower than approximately 6 g/10 min, ideally lower than 5.5 g/10 min. This upper limit is desirable because it appears that a viscosity that is too low is accompanied by a rise in the brittleness temperature. The lower limit, for its part, is not critical and its only purpose is to improve the workability of the compositions: in general, copolymers (B) whose MFI is at least approximately 4 g/10 min are chosen. For this same reason of workability, homopolymers whose MFI (under the same conditions) is at least approximately 0.5 g/10 min are generally employed. In fact, the MFI of the homopolymer employed in a composition of the invention is essentially chosen as a function of the MFI of the copolymer and of the relative proportions of these two types of polymers, so that the MFI of the composition preferably does not exceed approximately 1 g/10 min (under the abovementioned conditions of measurement).
The addition of an appropriate xe2x80x9clow temperaturexe2x80x9d plasticizer to the homopolymer-thermoplastic copolymer mixture, in proportions which are generally of at least approximately 5% [relative to the total weight of the polymers (A) and (B), that is to say more than 5 kg of plasticizer per 100 kg of unplasticized polymeric material], preferably of at least 8%, and not exceeding approximately 20%, preferably 15%, makes it possible to improve the low-temperature behaviour of the compositions. A proportion of plasticizer of approximately 10% gives highly satisfactory results. A plasticizer which has shown itself to be particularly advantageous within the context of the present invention is DBS (dibutyl sebacate: C4H9xe2x80x94COOxe2x80x94(CH2)8xe2x80x94COOxe2x80x94C4H9). Other effective plasticizers are polymeric polyesters such as those derived from adipic, azelaic or sebacic acids and diols, and their mixtures, but on condition that their molecular mass is at least approximately 1500, preferably at least 1800, and not exceeding approximately 5000, preferably lower than 2500. Polyesters of excessively high molecular mass result, in fact, in compositions of lower impact strength.
The DBS and the abovementioned polyesters are incorporated without any difficulty in the mixtures of fluoropolymers (A) and (B) defined above and produce compositions whose impact strength is satisfactory.
It has been found, furthermore, that the compositions of the invention resist deplasticization much better than a plasticized single homopolymer.
The compositions of the invention may additionally contain conventional additives such as filling materials, pigments, fibre reinforcements, electrically conductive particles and the like.
The compositions of the invention can be obtained by any of the usual techniques for preparing polymeric compositions, especially by premixing the various polymers in the form of powders or granules (as well as optionally with the other additives or filling materials) in the desired proportions, before they are subjected to a thermomechanical processing technique such as extrusion, injection moulding and the like.
This operating method can be applied either with a view to manufacturing finished products such as, for example, pipes, or, by adding a granulation stage thereto, with a view to having available granules containing the desired polymers, additives and fillers in the suitable proportions, thus facilitating a subsequent conversion into finished products.
For the reasons set out above, the compositions according to the invention show themselves to be particularly suitable for the manufacture, especially by extrusion, of pipes subjected to severe use conditions, in particular pipes intended for conveying hot hydrocarbons under pressure, but this represents only one of their potential applications, other articles manufactured from these compositions forming the subject of the present invention just as well as pipes.
The present invention is also aimed at providing pipes and other articles with a number of layers at least one of which consists of a composition in accordance with the invention. Such multilayer articles can be manufactured by many known techniques, such as coextrusion.